Magnetorheological (MR) fluid is composed of magnetizable microparticles (e.g., 1-10 microns in diameter) that are suspended in a carrier oil. Under a strong magnetic field of 0.1 to 1 Tesla, the microparticles become magnetized and spontaneously bond together to form semi-rigid chains. This network of bonded microparticles causes the fluid to harden into a solid. Solidification is reversible and the MR fluid instantly returns to the liquid state once the field is removed.
This property of dramatic but reversible field-controlled phase change makes MR fluid attractive for applications that require tunable rigidity and viscosity. Currently, MR-fluid is used in clutches, brakes, and suspension for high-end automotive systems. There is also interest in using MR-fluid for tunable damping in active orthotics and prosthetics. In both applications, MR-fluid functions as a lubricant that controls the friction between sliding surfaces. In the absence of a magnetic field, the lubricant has low viscosity and the surfaces slide freely past each other. When a strong magnetic field is applied, the fluid hardens and prevents sliding by bonding the surfaces together like a plastic glue.
Actively controlling material impedance is enabling for emerging fields such as soft robots for exploration and natural disaster relief and soft active orthotics for motor therapy and gait correction. Recent efforts have focused on jamming techniques such as pneumatic-controlled packing of granular particles in an elastomer-sealed chamber and the hydration of a soft nanowhisker-gel composite, which solidifies when dry by forming rigid, cellulose networks. Compliance control has also been accomplished with mechanisms that utilize, gears, pulleys, motors, and springs. Designs have been inspired by a variety of systems in nature, including catch connective tissue in sea cucumbers and muscular co-contraction in human motor tasks.
Methods based on fluidic jamming and springs require the added complexity of external pumps, tanks, and motors and, hence, may not be suitable in low power or millimeter scale systems. One promising alternative is to use magnetorheological (MR) fluid, which solidifies in the presence of a strong magnetic field. However, current magnetorheological-based methods for active damping and stiffness control require a relatively large magnetic field to solidify the fluid and resist interfacial sliding of discs and clutches. For example, a 0.5-1 Tesla (T) magnetic field may be required in the aforementioned application. While appropriate for large-scale automotive and industrial applications, such techniques are prohibitive in small and low power systems where magnetic field generation is limited.
Increasingly used in automotives and aerospace, MR-fluid has also been utilized for stiffness and damping control in other applications such as adaptive orthotic devices. However, due to solidification requirements commanding relatively large magnetic fields (e.g., 0.5-1 Tesla), practical application has been prohibitive for small or low power devices.
Thus, exploiting properties of MR-fluid for soft, miniature, and low-powered systems requires novel insights and mechanisms for reversible stiffness control at low or moderate magnetic fields (1-10 mT, 1 mT=10−3 Tesla). This is because strong magnetic fields of greater than 100 mT require either permanent magnets (which cannot be switched off) or large electromagnets that are rigid and consume great amounts of both space and electrical power.
Accordingly, a need exists for methods and systems having adaptive functionality aided by novel, non-solidification modes of MR compliance control.